Wafer-to-wafer (W2W) bonding is deployed in a wide range of semiconductor process applications for forming semiconductor devices. Examples of semiconductor process applications where wafer-to-wafer bonding is applied include substrate engineering and fabrication of integrated circuits, packaging and encapsulation of micro-electro-mechanical-systems (MEMS) and stacking of many processed layers (3D-integration) of pure microelectronics. W2W bonding involves aligning the surfaces of two or more wafers, transporting the aligned wafers into a wafer bonding chamber, bringing the wafer surfaces in contact and forming a strong bond interface between them. The overall process yield and manufacturing cost of the so produced semiconductor devices and ultimately the cost of the electronic products that incorporate these devices depend greatly upon the quality of the wafer-to-wafer bond. The quality of the W2W bond depends upon the accuracy of the wafer alignment, the preservation of the wafer alignment during the transport and the bonding process, and the uniformity and integrity of the bond strength across the wafer bond interfaces. Furthermore, extreme care is needed during the transport, positioning, centering and alignment of the wafers in order to avoid fracture, surface damage, or warping of the wafers.
FIG. 1A depicts a schematic diagram of a conventional transport fixture used to transport aligned wafers from an aligner to a bonder, in accordance with the prior art. Traditionally, a wafer pair 18 is aligned in an aligner station 50 and the aligned wafer pair 18 is secured onto a transport fixture 24, as shown in FIG. 1A. The transport fixture 24 carries the aligned wafer pair 18 to the bonding station 60 and to any further processing stations. A prior art transport fixture 24 is described in U.S. Pat. No. 7,948,034 issued on May 24, 2011 and entitled “APPARATUS AND METHOD FOR SEMICONDUCTOR BONDING”, the contents of which are expressly incorporated herein by reference.
FIG. 2A depicts the conventional transport fixture of FIG. 1A and as discussed relative to FIG. 3, in accordance with the prior art, and FIG. 2B depicts an enlarged view of the clamping assemblies of the conventional transport fixture of FIG. 2A, in accordance with the prior art. FIG. 3 is a schematic depiction of loading an aligned wafer pair into a bonding chamber using a conventional transport fixture, in accordance with the prior art. Referring first to FIG. 3, a conventional transport fixture 24 is sized to hold an aligned wafer pair (not shown) and a transport device 16 is used to move the transport fixture 24 and the aligned wafer pair into and out of the bonding chamber 12. In one example, transport device 16 is a transport arm or slide that is automated or otherwise manually operated.
As shown in FIG. 2A, transport fixture 24 is a circular shaped ring 280, often constructed from titanium, and includes three noses 280a, 280b, 280c that are symmetrically spaced about the circular shaped ring 280 that act as support points for a base wafer. Proximate to each of the three noses 280a, 280b, 280c are three spacer and clamp assemblies 282a, 282b, 282c arranged symmetrically at the periphery of the circular ring at 120 degrees apart. Each spacer and clamp assembly 282a, 282b, 282c includes a spacer 284 and a clamp 286. Spacer 284 is configured to set two wafers at a predetermined distance. Spacers with different thicknesses may be selected for setting different spacings between the two wafers. Once the spacers are inserted between the wafers, the clamp 286 is clamped down to lock the position of the two wafers. The clamp 286 may be a single structure or a linkage that moves downward to contact an upper wafer to retain it in position on the transport fixture 24. Each spacer 284 and each clamp 286 are independently activated by linear actuators 283 and 285, respectively.
For the bonding process, two aligned wafers are placed in the carrier fixture 24 and are spaced apart with spacers 284 and then clamped down with clamps 286. The fixture with the clamped wafers is inserted in the bonding chamber 12 and then each clamp 286 is unclamped one at a time, and the spacers 284 are removed. Once all spacers 284 are removed and the two wafers are staked together with a pneumatically controlled center pin. Then, a force column is applied to facilitate the bonding process in the bonding device 12 throughout the high-temperature bonding process.
Within the industry, it is known that the transport fixtures 24 can be heavy and challenging for the transport device 16 or a robot to handle. Further, once they are positioned within the bonding device 12, the transport fixtures 24 remain in the bonding device 12 throughout the duration of the bonding process, thus subjecting the transport fixtures 24 to bonding environments of up to 550° C. temperatures, as well as chamber gasses and/or pressures that may be used within the bonding device 12. In particular, the transport fixture 24 may be positioned for an hour or more in a location only a few millimeters away from an outer circumference of a heated chuck of the bonding device 12, such that the transport fixture 24 gets very hot. These conditions place a significant amount of stress on the transport fixtures 24, and especially on the intricate mechanics of the spacers 284 and clamps 286. As a result, over time, the transport fixtures 24 become unreliable and require significant servicing including sensitive adjustment of the mechanics, which has high costs and takes substantial time.
In other implementations, the aligned wafer pair is bonded temporarily and the temporarily bonded wafer pair is transported into the bonding station and any other processing stations. Temporary bonding of the wafers may be used to minimize alignment shift error during processing. The temporary wafer bonding techniques include bonding the centers or the edges of the wafers with a laser beam, bonding the centers or the edges of the wafers with a temporary tack adhesive and bonding the centers or the edges of the wafers via hybrid fusion. The bonded wafer pair is then transported to the bonding device with a transport fixture or similar, conventional transportation devices. The laser bonding techniques require at least one laser-transparent wafer and the adhesive bonding techniques may contribute to contamination of the wafer surfaces.
Accordingly, in light of the aforementioned deficiencies and inadequacies, it is desirable to provide an industrial-scale device for handling precisely aligned and centered semiconductor wafer pairs for wafer-to-wafer bonding applications with high throughput and the ability to handle all types of wafers without introducing any contaminants.